Medical device companies could someday use 3-D printers to quickly make an array of products from custom implants to valves and ultra fine micro-needle arrays for drug delivery. But the technology is not quite ready for prime time, said an expert in the field. A handful of current 3-D printing techniques all hold promise and pitfalls. The systems are typically much faster and simpler than conventional manufacturing techniques, but they are still relatively imprecise, expensive and lack bio-compatibility, said Roger Narayan, professor of biomedical engineering at the University of North Carolina at Chapel Hill